Method of controlling anti-lock brake system for vehicles and method of finding control point in ABS

ABSTRACT

In an anti-lock brake system mounted on a vehicle wherein when the vehicle is braked in an emergency during running, an increase in the road surface friction force or road surface friction coefficient owing to an increase in the brake pressure is detected by a road surface friction force detecting device or road surface friction coefficient detecting device. The optimum control start point associated with an increase in the signal value of the road surface friction force F or road surface friction coefficient μ provided by the road surface friction force detecting device or road surface friction coefficient detecting device is decided by using a decrease in the wheel speed, i.e., by using the wheel speed ω or dω/dt. Thereafter, from the point where the specified value of control based on F or μ, or dF/dt or dμ/dt, the brake pressure is moved from the pressure increasing mode to the pressure retaining or decreasing mode.

BACKGROUND OF THE INVENTION

A first form of this invention relates to a method of controlling ananti-lock brake system for vehicles which prevents the locking of thewheels upon emergency braking of a vehicle, the method beingcharacterized in that it uses the road surface friction force F or roadsurface friction coefficient μ, instead of slip factor used in the priorart, to control the system.

A second form of this invention relates to a method of finding a controlpoint in an anti-lock brake system (ABS) for vehicles which prevents thelocking of the wheels upon emergency braking of a vehicle, the methodmakes an errorless ABS control decision by using a road surface frictionforce detecting sensor or a road surface friction coefficient detectingsensor.

Method of Controlling Anti-Lock Brake System for Vehicles

Generally, conventional anti-lock brake systems for vehicles, e.g.,automobiles, automatically control the brake operation such that theslip ratio falls in a given range on the basis of the vehicle speed andwheel speed (e.g., Japanese Patent Publication No. 30585/1984 andJapanese Patent Application Laid-Open Specification No. 61354/1985). Therelation between road surface friction coefficient μ, and slip ratio canvary depending on the road surface conditions and for this reason thesystems sometimes fail to provide a maximum brake pressure, in whichcase a minimum brake distance cannot be obtained. Further, since thevehicle speed is a value estimated from the wheel speed, there is aproblem involving precision in the control of slip ratio. To accuratelyknow the vehicle speed, there is a need for a complicated device, suchas a ground-relative speed sensor (e.g., Japanese Patent ApplicationLaid-Open Specification No. 64861/1988) or a vehicle deceleration sensor(e.g., Japanese Patent Application Laid-Open Specification No.170157/1988). In the device described in Japanese Patent ApplicationLaid-Open Specification No. 25169/1988, the torque of road surfacefriction force (tire torque) acting on the wheel is found by calculationfrom wheel angular acceleration and brake liquid pressure and that valueof the tire torque at which the tire torque starts to decrease duringthe increase of the brake liquid pressure is employed as one of thefactors for deciding the conditions immediately before the locking ofthe wheel. However, in this device, the tire torque is indirectly foundby calculation from the wheel angular acceleration and brake liquidpressure and on account of the presence of uncertain constants such asbrake efficiency and the moment of inertia of the wheel, there is aproblem on precision. Further, since the pneumatic pressure in the tireof the wheel and the distance from the ground to the vehicle vary, thereis also a problem that the ratio between the road surface friction forceand the tire torque is not always maintained at a constant value.

In order to eliminate the drawback inherent in the conventional devicedescribed above, the present applicant has previously proposed, inJapanese Patent Application No. 197809/1989 (Japanese Patent ApplicationLaid-open Specification No. 220056/1991), an anti-lock brake system forvehicles, comprising a strain gauge disposed in the vicinity of theaxle, a load surface friction force detecting device having means fordirectly measuring shearing strains in the vicinity of the axle, and avertical load detecting device, and means whereby in response to anoutput signal from a road surface friction coefficient detecting devicehaving means for arithmetically processing detection signals from thetwo devices, the brake pressure is increased when the road surfacefriction force or road surface friction coefficient increases withincreasing brake pressure or it is decreased when the road surfacefriction force or road surface friction coefficient decreases despiteincreasing brake pressure, and if the road surface friction force orroad surface friction coefficient decreases with decreasing brakepressure, the brake pressure is increased again, such operations beingrepeated.

In the case where anti-lock brake control for vehicles is effected byusing the system, it has been found that owing to disturbance sourcessuch as vibrations of the tire and road surface during brake operationand the suspension, the signal value of the road surface friction forceF or road surface friction coefficient μ sometimes fluctuates in acertain range, causing the accurate control start point to be mistaken.

Method of Detecting Control Point in ABS

Generally, conventional anti-lock brake systems for vehicles, e.g.,automobiles, automatically control the brake operation such that theslip ratio falls in a given range on the basis of the vehicle speed andwheel speed (e.g., Japanese Patent Publication No. 30585/1984 andJapanese Patent Application Laid-Open Specification No. 61354/1985). Therelation between road surface friction coefficient and slip ratio canvary depending on the road surface conditions and for this reason thesystems sometimes fail to provide a maximum brake pressure, in whichcase a minimum brake distance cannot be obtained. Further, since thevehicle speed is a value estimated from the wheel speed, there is aproblem on precision in the control of slip ratio. To accurately knowthe vehicle speed, there is a need for a complicated device, such as aground-relative speed sensor (e.g., Japanese Patent ApplicationLaid-Open Specification No. 64861/1988) or a vehicle deceleration sensor(e.g., Japanese Patent Application Laid-Open Specification No.170157/1988). In the device described in Japanese Patent ApplicationLaid-Open Specification No. 25169/1988, the torque of road surfacefriction force (tire torque) acting on the wheel is found by calculationfrom wheel angular acceleration and brake liquid pressure and that valueof the tire torque at which the tire torque starts to decrease duringthe increase of the brake liquid pressure is employed as one of thefactors for deciding the conditions immediately before the locking ofthe wheel. However, in this device, the tire torque is indirectly foundby calculation from the wheel angular acceleration and brake liquidpressure and on account of the presence of uncertain constants such asbrake efficiency and the moment of inertia of the wheel, there is aproblem on precision. Further, since the pneumatic pressure in the tireof the wheel and the distance from the ground to the vehicle vary, thereis also a problem that the ratio between the road surface friction forceand the tire torque is not always maintained at a constant value.

In order to eliminate the drawback inherent in the conventional devicedescribed above, the present applicant has previously proposed, inJapanese Patent Application No. 197809/1989 (Japanese Patent ApplicationLaid-open Specification No. 220056/1991), an anti-lock brake system forvehicles, comprising a strain gauge disposed in the vicinity of theaxle, a load surface friction force detecting device having means fordirectly measuring shearing strains in the vicinity of the axle, and avertical load detecting device, and means whereby in response to anoutput signal from a road surface friction coefficient detecting devicehaving means for arithmetically processing detection signals from thetwo devices, the brake pressure is increased when the road surfacefriction force or road surface friction coefficient increases withincreasing brake pressure or it is decreased when the road surfacefriction force or road surface friction coefficient decreases despiteincreasing brake pressure, and if the road surface friction force orroad surface friction coefficient decreases with decreasing brakepressure, the brake pressure is increased again, such operations beingrepeated.

In the case where anti-lock brake control for vehicles is effected byusing the above system, it has been found that owing to crosstalk suchas brake torque contained in sensor signals, the accurate control startpoint can often be mistaken.

SUMMARY OF THE INVENTION

Method of Controlling Anti-Lock Brake System for Vehicles

In view of the problem inherent in the anti-lock brake system forvehicles according to the prior art described above, the presentinvention has for its object the provision of a method of controlling ananti-lock brake system for vehicles, the method being improved to effectstabilized control by eliminating disturbance sources in the vicinity ofthe optimum control start point concomitant with the increase of thesignal value of the road surface friction force F or road surfacefriction coefficient μ or by confining disturbance sources associatedwith the optimum control start point in a range of given width, themethod preventing the brake pressure from being unnecessarily decreased.

In accordance with a first embodiment there is provided a method ofcontrolling an anti-lock brake system for vehicles wherein when avehicle having an anti-lock brake system mounted thereon has theemergency brake applied thereto, a change in the road surface frictionforce or road surface friction coefficient due to the increased brakepressure is detected by a road surface friction force detecting deviceor a road surface friction coefficient detecting device, the methodbeing characterized in that to decide the optimum control start pointconcomitant with the increase of the signal value of the road surfacefriction force F or road surface friction coefficient μ detected by theroad surface friction force detecting device or road surface frictioncoefficient detecting device, use is made of means for eliminatingdisturbance sources which impede such decision. Accordingly, eliminatingdisturbance sources in the vicinity of the optimum control start pointconcomitant with the increase of the signal value of the road surfacefriction force F or road surface friction coefficient μ or confiningdisturbance sources associated with the optimum control start point in arange of given width ensures that there is no unnecessary decrease inbrake pressure and that efficient and stabilized control is effected.

In accordance with a second embodiment there is provided a method ofcontrolling an anti-lock brake system for vehicles, wherein when avehicle having an anti-lock brake system mounted thereon has theemergency brake applied thereto, a change in the road surface frictionforce or road surface friction coefficient due to the increased brakepressure is detected by a road surface friction force detecting deviceor a road surface friction coefficient detecting device, the methodbeing characterized in that the optimum control start point concomitantwith the increase of the signal value of the road surface friction forceF or road surface friction coefficient μ due to the increased brakepressure detected by the road surface friction force detecting device orroad surface friction coefficient detecting device is decided by a dropin the wheel speed, that is, by using the wheel speed or dω/dt, and thenfrom the point where the specified value of control by F or μ or bydF/dt or dμ/dt is reached, the brake pressure is shifted from thepressure increasing mode to the pressure retaining or decreasing mode,so as to eliminate disturbance sources. Accordingly, a drop in the wheelspeed is decided by the wheel speed ω or dω/dt and the optimum controlstart point concomitant with the increase of the signal value of theroad surface friction force F or road surface friction coefficient μ isspecified, thereby providing the optimum control start point free fromthe influences of disturbance sources during ABS control.

In accordance with a third embodiment there is provided a method ofcontrolling an anti-lock brake system for vehicles wherein when avehicle having an anti-lock brake system mounted thereon has theemergency brake applied thereto, a change in the road surface frictionforce or road surface friction coefficient due to the increased brakepressure is detected by a road surface friction force detecting deviceor a road surface friction coefficient detecting device, the methodbeing characterized in that the optimum control start point concomitantwith the increase of the signal value of the road surface friction forceF or road surface friction coefficient μ due to the increased brakepressure detected by the road surface friction force detecting device orroad surface friction coefficient detecting device is decided by a dropin the vehicle acceleration decided by dV/dt using an accelerationsensor, and then from the point where the specified value of control byF or μ or by dF/dt or dμ/dt is reached, the brake pressure is shiftedfrom the pressure increasing mode to the pressure retaining ordecreasing mode, so as to eliminate disturbance sources. Accordingly, adrop in the vehicle acceleration is decided by dV/dt using anacceleration sensor and then the optimum control start point concomitantwith the increase of the road surface friction force F or road surfacefriction coefficient μ is designated, thereby providing the optimumcontrol start point free from the influences of disturbance sourcesduring ABS control.

In accordance with a fourth embodiment, a method of controlling ananti-lock brake system for vehicles is provided wherein when a vehiclehaving an anti-lock brake system mounted thereon has the emergency brakeapplied thereto, a change in the road surface friction force or roadsurface friction coefficient due to the increased brake pressure isdetected by a road surface friction force detecting device or a roadsurface friction coefficient detecting device, the method beingcharacterized in that the optimum control start point concomitant withthe increase of the signal value of the road surface friction force F orroad surface friction coefficient μ detected by the road surfacefriction force detecting device or road surface friction coefficientdetecting device is decided by both the wheel speed ω or dω/dt anddV/dt, and then from the point where the specified value of control by For μ or by dF/dt or dμ/dt is reached, the brake pressure is shifted fromthe pressure increasing mode to the pressure retaining or decreasingmode, so as to eliminate disturbance sources. According to the fourthembodiment, a drop in the wheel speed is decided by the wheel speed ω ordω/dt and a drop in the vehicle acceleration is decided by dV/dt usingan acceleration sensor and then the optimum control start pointconcomitant with the increase of the road surface friction force F orroad surface friction coefficient μ is designated, thereby providing theoptimum control start point free from the influences of disturbancesources during ABS control.

In accordance with the second through fourth embodiments, a drop in thewheel speed during brake operation is detected by the specified value ofω or by dω/dt so as to provide a control start point or a drop in thevehicle speed is detected by dV/dt so as to provide a control startpoint or detected by both to be decided as the control start point;therefore, the optimum control start point is obtained without beinginfluenced by disturbance sources such as vibrations between the tireand road surface during brake operation and the suspension, thusenabling accurate control to be effected using F or μ.

In accordance with a fifth embodiment, a method of controlling ananti-lock brake system for vehicles according to the first throughfourth embodiments is further characterized in that in the pressuredecreasing mode subsequent to the retaining mode or after moving to thepressure decreasing mode, the brake pressure is shifted from thepressure decreasing mode to the pressure increasing mode by a pressuredecreasing threshold value, a set value of a specified value of elapsedtime or a decision circuit or by a combination of two or more of suchfactors, so as to eliminate disturbance sources. According to the fifthembodiment, any one of the above control methods of the first throughfourth embodiments is used to shift the brake pressure from the pressureincreasing mode to the retaining mode or pressure decreasing mode andthen the brake pressure is shifted from the pressure decreasing mode tothe pressure increasing mode by a pressure decreasing threshold value, aset value of a specified value of elapsed time or a decision circuit orby a combination of two or more of such factors, so as to eliminatedisturbance sources, thus effecting ABS control.

According to a sixth embodiment, a method of controlling an anti-lockbrake system for vehicles, characterized in that during travel of avehicle having an anti-lock brake system, the first brake pressurecontrol is effected by using any of the control methods of the firstthrough fourth embodiments, and then at the point of time when thecontrol method of the fifth embodiment is completed, the control iscontinuously repetitively effected by using any of the control methodsof the first through fourth embodiments or the fifth embodiment, untilthe vehicle stops or in and after the second time the control iscontinuously repetitively effected by using F or μ, or dF/dt or dμ/dtalone and the control method of the fifth embodiment using the pressuredecreasing threshold value until the vehicle stops, thus eliminatingdisturbance sources.

According to the invention of the sixth embodiment, the first brakepressure control is effected by using any of the above control methodsof the first through fourth embodiments, and then after the brakepressure has been shifted from the pressure increasing mode to theretaining or pressure decreasing mode, the brake pressure is shiftedfrom the pressure decreasing mode to the pressure increasing mode by apressure decreasing threshold value, a set value of a specified value ofelapsed time or a decision circuit or by a combination of two or more ofsuch factors, whereupon the control is continuously repetitivelyeffected until the vehicle stops or after the second time the control iscontinuously repetitively effected using F or μ, or dF/dt or dμ/dt aloneand the control method of the fifth embodiment until the vehicle stops.

According to the fifth and sixth embodiments, after any of theoperations described in the second through fourth embodiments has beenperformed and the brake pressure has been shifted to the pressuredecreasing mode, the control is continuously repetitively effected inwhich the brake pressure is shifted from the pressure decreasing mode tothe pressure increasing mode by a pressure decreasing threshold value, aset value of a specified value of elapsed time or a decision circuit orby a combination of two of such factors, or in and after the second timethe control is continuously repetitively effected using F or μ or dF/dtor dμ/dt alone and the aforesaid control method; therefore, the shift ofthe brake pressure from the pressure decreasing mode to the pressureincreasing mode is continuously repetitively effected until the vehiclestops, so that disturbance in the vicinity of the F or μ brake optimumvalue can be eliminated.

According to a seventh embodiment, a method of controlling an anti-lockbrake system for vehicles is provided wherein when a vehicle having ananti-lock brake system mounted thereon has the emergency brake appliedthereto, a change in the road surface friction force or road surfacefriction coefficient due to the increased brake pressure is detected bya road surface friction force detecting device or a road surfacefriction coefficient detecting device, the method being characterized inthat after the maximum value of the signal value of the road surfacefriction force F or road surface friction coefficient μ detected by theroad surface friction detecting device or road surface frictioncoefficient detecting device has been ascertained, fixed lower limitsare provided for the approximate maximum F value or approximate maximumμ value and the maximum F value or maximum μ value and the brakepressure is controlled such that F or μ is stably retained in the range,thus eliminating disturbance sources. Using the upper and lower limits,the method is not influenced by fluctuations in the F or μ value causedby disturbance and the brake pressure maintains approximately themaximum F value or maximum μ value without any unnecessary decrease inbrake pressure, so that efficient and stabilized control can beeffected.

An eighth embodiment of the invention includes a method of controllingan anti-lock brake system for vehicles wherein when a vehicle having ananti-lock brake system mounted thereon has the emergency brake appliedthereto, a change in the road surface friction force or road surfacefriction coefficient due to the increased brake pressure is detected bya road surface friction force detecting device or a road surfacefriction coefficient detecting device, the method being characterized inthat after the maximum value of the signal value of the road surfacefriction force F or road surface friction coefficient μ detected by theroad surface friction detecting device or road surface frictioncoefficient detecting device has been ascertained, fixed lower limitsare provided for the approximate maximum F value or approximate maximumμ value and the maximum F value or maximum μ value and the brakepressure is controlled by being increased or decreased in the range inwhich F or μ is retained, thus eliminating disturbance sources.Accordingly, the brake pressure is retained between the maximum F valueor maximum μ value and the F value or μ value which is in thedisturbance range, and efficient and stabilized control is effectedwithout any unnecessary decrease in brake pressure.

According to the seventh and eighth embodiments, in contrast to the factthat in the conventional control method, in order to prevent the lockingof the wheel in the vicinity of the limit of brake force in thepossession of a road surface, it has been necessary to once decrease thebrake pressure to decrease the brake force on the road surface, thepresent invention makes it unnecessary to decrease the pressure,enabling the brake pressure to be retained in the vicinity of themaximum value of brake force in the possession of the road surface.Thereby, the brake distance for the emergency brake can be decreased,improving the safety of the vehicle.

A ninth embodiment includes a method of controlling an anti-lock brakesystem for vehicles, characterized in that in the seventh and eighthembodiments, variations in F or μ with respect to brake pressure aremonitored and when the upper limit of brake pressure in a preset controlrange is reached, if F or μ exceeds the previous maximum F or maximum μ,the brake pressure is further increased to ascertain the maximum μagain, where after the above-mentioned brake pressure control iseffected, thus eliminating disturbance. During control according to thecontrol method of the seventh and eighth embodiments using upper andlower limits, if the F value or μ value obtained from the road surfaceincreases, that is, when the vehicle moves from a slippery road surfaceto a less slippery road surface, the brake force obtained from the roadsurface is efficiently used to shorten the brake distance.

According to the ninth embodiment, in contrast to the fact theconventional control method has no means for knowing the vehicle havingmoved to a road surface which provides a higher road surface brake forceduring anti-lock brake operation, the change of the road surface can bedetected with good responsiveness to retain the optimum brake pressure.Thereby, the road surface brake force which can be primarily acquired iseffectively used to shorten, thus improving the safety of the vehicle.

According to a tenth embodiment, a method of controlling an anti-lockbrake system for vehicles is provided, uses the above upper and lowerlimit methods and, characterized in that in the methods of the seventhand eighth embodiments, variations in F or μ with respect to brakepressure are monitored and if the F value or μ value decreases withrespect to the brake pressure in a preset brake pressure control range,the brake pressure is rapidly decreased to ascertain the maximum μagain, thus effecting control to eliminate disturbance. According to theembodiment of the tenth embodiment during control according to thecontrol method of the seventh and eighth embodiments, if the F value orμ value obtained from the road surface decreases, that is, when thevehicle moves a more slippery road surface, the brake force is rapidlydecreased to avoid locking and the optimum brake pressure control on theroad surface in question is effected.

According to the tenth embodiment, in contrast to the fact that theconventional control method makes the detection only when the wheelactually starts to be locked, the detection is made in the stage wherethe wheel is supposed to start to be locked, making it possible to startadjusting the brake pressure in the early period, thus eliminating theneed for adjusting excessive brake pressure resulting from delayeddecision, so that the optimum brake pressure control can be effected.

An eleventh embodiment provides a method of controlling an anti-lockbrake system for vehicles, characterized in that in the case where themaximum value of the signal value of the road surface friction force For road surface friction coefficient μ detected by the road surfacefriction force detecting device or road surface friction coefficientdetecting device can hardly be ascertained, quasi-F or quasi-μ is foundbetween the maximum F value or maximum μ value and the minimum F valueor minimum μ value within a given period of time, and the brake pressurecontrol is effected corresponding to such quasi-F or quasi-μ, thuseliminating disturbance sources. Therefore, stabilized brake pressurecontrol can be effected even on a road surface which causes vigorousvibrations, such as an undulating road surface.

A twelfth embodiment further provides a method of controlling ananti-lock brake system for vehicles using the above quasi-F and quasi-μmethods of the eleventh embodiment, where in the case where the valuebetween the maximum F value or maximum μ value and the minimum F valueor minimum μ value within a given period of time varies beyond theallowable range, the individual values are measured again within a givenperiod of time to newly find quasi-F or quasi-μ and the brake pressurecontrol is effected corresponding to such quasi-F or quasi-μ, thuseliminating disturbance sources. Therefore, optimum brake pressurecontrol can be effected even on a road surface which causes vigorousvibrations, such as an undulating road surface.

A thirteenth embodiment further provides a method of controlling ananti-lock brake system for vehicles, using the above quasi-F and quasi-μmethods of the eleventh and twelfth embodiments, where in the case wherethe maximum μ can be ascertained, control is effected according to thecontrol methods described above using fixed limits of the sevenththrough tenth embodiments thus eliminating disturbance sources. Thus,effective stabilized anti-lock brake control can be effected even on aroad surface which causes vigorous vibrations, such as an undulatingroad surface.

Furthermore, according to the methods of the eleventh and thirteenthembodiments using quasi values, stabilized anti-lock brake control canbe effected even on a road surface where the detected value of F or μare high in variation, such as an undulating road surface.

Method of Detecting Control Point in ABS

In view of the above problems inherent in the anti-lock brake systemsfor vehicles according to the prior art, the invention has for itsobject the provision of a method of detecting a control point whichenables normal ABS control even if a sensor is subject to crosstalk orthe like is used. The present invention further provides the followingembodiments.

The invention includes a method of a fourteenth embodiment detecting acontrol point in an ABS having a stress sensor which provides an outputproportional to the road surface friction F or road surface frictioncoefficient μ having mixed therein a crosstalk component, such as braketorque T, and to the brake torque T, the method being characterized inthat it uses adjusting means for making adjustment from the rise of thebrake start such that detected signals of F or μ and T are adjusted inratio or made equal in value, and decision means for deciding thecontrol point by a change in the ratio or in adjustment coefficient,wherein the timing of the control point is detected by the size of achange in the ratio of detected signals of F or μ and T or in adjustmentcoefficient. According to the fourteenth embodiment, in order to detectthe timing of the control point, the detected signals of F or μ and Tare adjusted in ratio or made equal in value from the rise of the brakestart and the control point is calculated by the size of the change inratio or in adjustment coefficient; thus, the control point which is notinfluenced by crosstalk or the like can be obtained.

The invention further provides is a method of a fifteenth embodimentdetecting a control point in an ABS, characterized in that in thedecision means of the fourteenth embodiment using the ratio, a point intime when the ratio of F or μ and T or adjustment coefficientsubstantially stops changing is decided to be the control timing.Thereby, the control point which is not influenced by crosstalk or thelike can be obtained.

The invention still further provides a method of a sixteenth embodimentdetecting a control point in an ABS, characterized in that in thedecision means the fourteenth embodiment using the above ratio, a, thepoint in time when the ratio of F or μ and T or the adjustmentcoefficient, during brake pressure decreasing control, becomessubstantially equal to the value obtained upon completion of thepreceding brake pressure decreasing control is decided as the completionof the brake pressure decreasing control, the point in time beingdecided to be the optimum control timing for ABS. Thereby, the controlpoint which is not influenced by crosstalk or the like can be obtained.

The invention also provides a method of a seventeenth embodiment ofdetecting a control point in an ABS, characterized in that in thedecision means using the above ratio, a point in time when the ratio ofF or μ and T or the adjustment coefficient, during the brake pressuredecreasing control, becomes above the value obtained during the firstbrake pressure decreasing control is decided to be the completion of thebrake pressure decreasing control, the point in time being decided to bethe optimum control timing for the ABS. Thereby, the control point whichis not influenced by crosstalk or the like can be obtained.

The invention further includes a method an eighteenth embodiment ofdetecting a control point in an ABS, characterized in that in thedecision means using the above ratio, a point of time when the ratio ofF or μ and T or the adjustment coefficient, during the brake pressuredecreasing control, starts to increase is decided to be the completionof the brake pressure decreasing control, the point in time beingdecided to be the optimum control timing for the ABS. Thereby, thecontrol point which is not influenced by crosstalk or the like can beobtained.

The invention still further includes a method of a nineteenth embodimentof detecting a control point in an ABS, characterized in that in thedecision means of the fourteenth embodiment using the ratio, a point intime when the ratio of F or μ and T or the adjustment coefficient,except during the brake pressure decreasing control, becomes just abovethe value obtained during the first brake pressure decreasing control issaluted as the start of brake pressure decreasing control, the point intime being decided to be the optimum control timing for the ABS.Thereby, the control point which is not influenced by crosstalk or thelike can be obtained.

The invention yet still further a method of a twentieth embodiment ofdetecting a control point in an ABS, characterized in that in thedecision means of the fourteenth embodiment a point in time when theratio of F or μ and T or the adjustment coefficient, during the brakepressure retaining control, becomes greater than the value obtainedduring the first brake pressure decreasing control is decided to be thestart of brake pressurization control, the point being decided to be theoptimum control timing for the ABS. Thereby, the control point which isnot influenced by crosstalk or the like can be obtained.

The invention also provides, a method of a twenty first embodiment ofdetecting the control point in an ABS, characterized in that in thedecision means of the fourteenth embodiment using the ratio, a point intime when the ratio of F or μ and T or the adjustment coefficient,during the brake pressure retaining control, becomes smaller than thevalue obtained during the first brake pressure decreasing control andbecomes further smaller is decided to be the start of brake pressuredecreasing control, the point being decided to be the optimum controltiming for the ABS. Thereby, the control point which is not influencedby crosstalk or the like can be obtained.

The invention additionally provides is a method of a twenty secondembodiment of detecting the control point in an ABS, characterized inthat in the decision means of the fourteenth embodiment using the ratio,a point in time when the ratio of F or μ and T or the adjustmentcoefficient, during the brake pressure retaining control, becomessmaller than the value obtained during the first brake pressuredecreasing control and is stabilized is decided to be continuation ofpressure retention or the start of control of gentle increase ofpressure, the point in time being decided to be the optimum controltiming for the ABS. Thereby, the control point which is not influencedby crosstalk or the like can be obtained.

According to the invention of the fourteenth through twenty secondembodiments, to detect the timing for control point, the detected signalof F or μ and T are adjusted in ratio or made equal in value from therise of brake start and the control point can be calculated from thesize of the change in the ratio or adjustment coefficient to provide thecontrol point which is not influenced by crosstalk or the like;therefore, using the detected value as such which has a crosstalkcomponent such as brake torque which is difficult to eliminate whendetecting the road surface friction force F or road surface frictioncoefficient μ, it is possible to find the accurate control point, and inABS control in which safety is the most important factor, theprobability of erroneous decision can be minimized.

The invention further provides a method of a twenty third embodiment ofdetecting the control point in an ABS having a stress sensor whichprovides an output proportional to the road surface friction F or roadsurface friction coefficient μ having mixed therein a crosstalkcomponent, such as brake torque T, and to the brake torque T, the methodbeing characterized in that it uses adjusting means for adjusting theratio of detected signals of F or μ and T, and decision means fordeciding the control point by a change in F-T value from its value afteradjustment, wherein the timing for the control point is detected by thechange in the F-T value. Accordingly, in order to detect the timing forthe control point, and T, the timing for the control point is calculatedand decided by a change in the F-T value during brake control and thusthe crosstalk component T, which is originally a noise, is positivelyutilized to provide the optimum control point.

The invention described further provides is a method of a twenty fourthembodiment of detecting the control point in an ABS having a stresssensor which provides an output proportional to the road surfacefriction force F or road surface friction coefficient μ and the braketorque T, the method being characterized in that the timing for thecontrol point is detected by a change in the F-T value during brakecontrol. Accordingly, in order to detect the timing for the controlpoint, and T, the timing for the control point is calculated and decidedby a change in F-T value during brake control and thus the crosstalkcomponent T, which is originally a noise, is positively utilized toprovide the optimum control point.

The invention still further provides a method of a twenty fifthembodiment of detecting the control point in an ABS, characterized inthat in the decision means of the twenty third and twenty fourthembodiments, a threshold value for the F-T value is provided to decidethe control point in control. Accordingly, to detect the timing for thecontrol point, a threshold value for the F-T value is provided duringbrake control to decide the timing in control or decide the timing forthe control of F, N, ΔF, ΔN, thereby eliminating the influences ofnoise.

The invention further includes a method of a twenty sixth embodiment ofdetecting the control point in an ABS, characterized in that in thethreshold value of the twenty fifth embodiment, a change in the F-Tthreshold value is corrected as the road surface friction coefficientchanges, thereby detecting the timing for the control point.Accordingly, in order to detect the timing for the control point, athreshold value is used for the F-T value to correct the threshold valuefor the F-T value as the road surface friction coefficient changes, soas to decide the timing for the control point; thus, as the vehiclespeed changes from high to low value, the threshold value is decreasedaccording as the road surface friction coefficient decreases, therebyproviding the optimum control point.

In the invention of the twenty third through twenty sixth embodiments,the T value which is originally a noise inherent in a sensor used in theinvention is used in the form of F-T, whereby without using a gear-likewheel speed sensor used in conventional ABSs, hybrid control can beeffected which uses both slip control with higher response and controldecision based on F or μ according to a second embodiment of theinvention. The present invention will now be described in more detailwith reference to embodiments thereof shown in the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an example of a hard arrangement for acontrol method for an anti-lock brake system for vehicles according to asecond embodiment of the present invention;

FIG. 2 is a block diagram showing an example of a hard arrangement for acontrol method for an anti-lock brake system for vehicles according to athird embodiment of the present invention;

FIG. 3 is a block diagram showing an example of a hard arrangement for acontrol method for anti-lock brake system for vehicles according to afourth embodiment of the present invention;

FIG. 4 is a block diagram showing an example of a hard arrangement for acontrol method for an anti-lock brake system for vehicles according tofifth and sixth embodiments of the present invention;

FIG. 5 is a flowchart showing a main routine processing of a controldevice shown in FIG. 4;

FIG. 6 is a flowchart showing a brake liquid pressure decreaseprocessing routine shown in FIG. 5;

FIG. 7 is a flowchart showing a brake liquid pressure re-increaseprocessing routine shown in FIG. 5;

FIG. 8 is a flowchart showing an interruption with respect to the mainroutine processing shown in FIG. 5;

FIG. 9 is a flowchart showing a control method for an anti-lock brakesystem for vehicles according to a seventh embodiment of the presentinvention;

FIG. 10 is a flowchart showing a control method for an anti-lock brakesystem for vehicles according to an eighth embodiment of the presentinvention;

FIG. 11 is a flowchart showing a control method for an anti-lock brakesystem for vehicles according to a ninth embodiment of the presentinvention;

FIG. 12 is a flowchart showing a control method for an anti-lock brakesystem for vehicles according to a tenth embodiment of the presentinvention;

FIG. 13 is a flowchart showing a control method on the basis of the roadsurface friction force F according to the seventh embodiment of thepresent invention;

FIG. 14 is a functional system diagram in the present invention; and

FIG. 15 is an output trend graph of road surface friction force, braketorque and brake oil pressure during hard braking.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Method of Controlling Anti-Lock Brake System for Vehicles

In accordance with a first embodiment there is provided a method ofcontrolling an anti-lock brake system for vehicles wherein when avehicle having an anti-lock brake system mounted thereon has theemergency brake applied thereto, a change in the road surface frictionforce or road surface friction coefficient due to the increased brakepressure is detected by a road surface friction force detecting deviceor a road surface friction coefficient detecting device, the methodbeing characterized in that to decide the optimum control start pointconcomitant with the increase of the signal value of the road surfacefriction force F or road surface friction coefficient μ detected by theroad surface friction force detecting device or road surface frictioncoefficient detecting device, use is made of means for eliminatingdisturbance sources which impede such decision. Accordingly, eliminatingdisturbance sources in the vicinity of the optimum control start pointconcomitant with the increase of the signal value of the road surfacefriction force F or road surface friction coefficient μ or confiningdisturbance sources associated with the optimum control start point in arange of given width ensures that there is no unnecessary decrease inbrake pressure and that efficient and stabilized control is effected.

FIG. 1 is a block diagram showing an example of a hard arrangement for acontrol method for an anti-lock brake system for vehicles using the roadsurface friction coefficient according to a second embodiment of thepresent invention. In the figure, the character A denotes an ω, {dotover (ω)} detecting device; 1 denotes a road surface frictioncoefficient μ detecting device; 2 denotes a brake pedal stepping-onforce sensor; 3 denotes a control device; 4 denotes a brake liquidpressure generating device; 5 denotes a brake device; and 6 denotes abrake liquid pressure detecting device. The control device 3 is composedof electronic circuits including a microprocessor, a memory and aninput/output interface, and is adapted to operates according to aprogram written in the memory in advance.

In the case where anti-brake system (ABS) control is effected using aroad surface friction coefficient μ the optimum control start point isoften misunderstood owing to disturbance sources such as vibrationsproduced in the tire and road surface during braking, and thesuspension. To eliminate this disturbance in the vicinity of the μcontrol optimum value to obtain the optimum control start point, in thisexample, the drop in the wheel speed owing to braking is detected by apredetermined value of ω or μ defined by$\mu = \frac{{\mathbb{d}\omega}/{\mathbb{d}t}}{N}$and using this as the control start point, the known ABS control methodis effected using μ.

As for the specified value of ω, in the case where the value at thestart of measurement is ω1 and the value at the specified time is ω2,the specified value is expressed by$\frac{{\omega\quad 1} - {\omega\quad 2}}{\omega\quad 2} \geq {0.03\quad{to}\quad 0.2}$Expressed using dω/dt, from the equation of wheel motion,Idω/dt=μ·N·r·T _(B)

In the above equation, I is the wheel inertia, N is the wheel load, r isthe wheel radius, T_(B) is the brake torque, and μ is the frictioncoefficient.

In addition, the brake torque T_(B) is found fromT _(B)=2·μ_(P) ·B·A·P _(w).

In this equation, μ_(P) is the friction coefficient between the brakedisk and pad, B is the effective radius of the pad, P_(w) is the brakeoil pressure.

Since I and r can be regarded as constants,$\mu \approx \frac{{{\mathbb{d}\omega}/{\mathbb{d}t}} + {TB}}{N}$and this μ is used.

The derivative dω/dt is detected by the wheel speed sensor now in use, Nis measured by a vertical load sensor mounted on the suspension, andT_(B) is calculated by the above equation. In addition, if the knowneliminating means is used, since the need for considering the crosstalkwith respect to the output from the F sensor measured as a sensorcomponent T_(B) by the μ sensor is decreased, μ can be calculated bythis equation. The proper range around this maximum value is taken asthe optimum control start point.

FIG. 2 is a block diagram showing an example of a hard arrangement for acontrol method for an anti-lock brake system for vehicles using the roadsurface friction coefficient μ a third embodiment of the presentinvention described, and this embodiment differs from the preceding onein that instead of the ω, {dot over (ω)} detecting device A shown inFIG. 1, use is made of a V detecting device A1 using an accelerationsensor, the rest of the arrangement being the same so that a repetitivedescription thereof is omitted. In this embodiment, to obtain theoptimum control start point, the acceleration sensor is used and as forμ according to dV/dt, from the equation of wheel motion,m·dV/dt=−μN

In the above equation, m is the vehicle weight, N is the wheel load, andμ is the friction coefficient. However, m can be regarded as a constant.

From the above equation,$\mu = {{- m}\frac{{\mathbb{d}V}/{\mathbb{d}t}}{N}}$and the μ in this equation is used.

And dV/dt is measured by the acceleration sensor and N is measured andcalculated by the vertical load sensor mounted on the suspension, andthe proper range around this maximum value is taken as the optimumcontrol start point.

FIG. 3 is a block diagram showing an example of a hard arrangement for acontrol method for anti-lock brake system for vehicles using the roadsurface friction coefficient μ of a fourth embodiment of the presentinvention, which differs from the embodiments shown in FIGS. 1 and 2only in that an ω, {dot over (ω)}, {dot over (v)} detecting device A2 isused, the rest of the arrangement being the same so that a repetitivedescription thereof is omitted.

FIG. 4 is a block diagram showing an example of a hard arrangement for acontrol method for an anti-lock brake system for vehicles using the roadsurface friction coefficient μ according to the fifth and sixthembodiments of the present invention, and this embodiment differs fromthe embodiments shown in FIGS. 1 through 3 in that detecting devices A,A1, A2 for ω, {dot over (ω)}, {dot over (v)} are selectively used and inthat a discriminating circuit B based on ω, {dot over (ω)}, {dot over(v)} is added to the control device 3, the rest of the arrangement beingthe same so that a repetitive description thereof is omitted.

The operation of the control device 3 in the embodiment shown in FIG. 4will now be described on the basis of the flowcharts shown in FIGS. 5through 8.

When the brake stepping-on force exceeds the preset value, the anti-lockbrake device starts to operate, changing the normal brake operation tothe anti-lock brake operation. The step 110 of the main routine shown inFIG. 5 represents the start of this anti-lock brake operation.Subsequently, at the step 111, the optimum control start point isdiscriminated on the basis of ω and {dot over (ω)} or {dot over (v)}, atthe step 112, the road surface friction coefficient μ is detected, andat the step 113, this value of μ is stored in a variable labeled byμt-1. Subsequently, at the step 114, this value is stored in a variablelabeled by this μp. Then, after the brake liquid pressure is increasedat the step 115, μ is detected at the step 116. At the step 117, thedetected value of μ of the step 116 is stored in a variable labeled byμt. Then the processing goes to the step 118, comparing the differenceμt-μt-1 between two values μt and μt-1 with a predetermined referencevalue μc. If the difference μt-μt-1 is greater than μc, the processinggoes to the step 119, and if it is equal to or smaller than μc, theprocessing goes to the brake liquid pressure decreasing routine at thestep 123. At the step 119, the value stored in the variable μt is storedin the variable μt-1 and this value of μt-1 is updated. subsequently theprocessing returns to the step 114.

At the brake liquid pressure decreasing routine 123, as shown in FIG. 6,first at the step 142 the brake liquid pressure is released or decreasedto a given lower level. Then at the step 143, the optimum control startpoint is discriminated on the basis of ω and {dot over (ω)} or {dot over(v)} and after the μ is detected at the step 144, this detected value isstored in the variable μt-1 at the step 145.

Then, the processing goes to the step 146, where it compares μt-1 withα·μp. The coefficient α is a constant preset at a suitable constantvalue in the range of 0 to 1. If the variable μt-1 is smaller, theprocessing goes to the step 149, where the brake liquid pressuredecreasing routine 123 is completed, and it goes to the brake liquidre-pressurizing routine at the step 124. If the variable μt-1 isgreater, it returns to the step 142.

At the brake liquid re-pressurizing routine subsequent to the brakeliquid pressure decreasing routine 123, the processing shown in FIG. 7is performed. First, at the step 162, the brake liquid pressure isincreased. Subsequently, at the step 163, the optimum control startpoint is discriminated on the basis of ω, {dot over (ω)} or {dot over(v)}, and μ is detected at the step 164 and is stored in the variable μtat the step 165. Then, the μt is compared with the variable μt-1 at thestep 166. If the variable μt is greater, the processing goes to the step167 where the variable μt is stored in the variable μt-1 to update thestored value of the variable μt-1. Then, the processing goes to the step171 to complete the brake liquid re-pressurizing routine, returning tothe step 114 of the main routine. If the variable μt is smaller or equalat the step 166, the processing goes to the step 168 to update thevariable μt-1 to the variable μt in the same manner as at the step 167.And the processing returns to the step 162.

As the control device 3 performs the above-described processing, theanti-lock brake system according to the present invention operates asfollows. When the anti-lock brake system starts to operate, while therate of increase of the road surface friction coefficient μ exceeds apredetermined reference value, the brake liquid pressure is kept on theincrease and ω or dω/dt or dV/dt is used to decide the optimum controlstart point, and when the rate of increase of the road surface frictioncoefficient μ lowers below the reference value, the brake liquidpressure is lowered or released. At this time, after moving to thepressure decreasing mode, the control is moved from the pressuredecreasing mode to the pressure increasing mode by using the thresholdvalue for pressure decrease, the preset value of specified value ofelapsed time, or the discriminating circuit or a combination of thesefactors. Thereafter, the above operation is continued and repeated untilthe vehicle stops.

In the case where the vehicle speed lowers below a given value duringthe anti-lock brake operation, no matter which step in the flowchartshown in FIGS. 5 through 7 the control device 3 is being performed, itimmediately executes the interruption routine shown in FIG. 8 to controlthe brake liquid pressure device such that it ends the anti-lock brakeoperation and returns to the normal brake operation. If the vehiclespeed is sufficiently low, there is no need for anti-lock brakeoperation and there is no need for it when the vehicle is stopping.

The above refers to an embodiment wherein the road surface frictioncoefficient μ is used, but in the case where the road surface frictionforce F is used, the same operation as in the above operation using μcan be performed by multiplying μ by N from the relationF=μ·N

FIG. 9, 9A is a schematic flowchart showing a control method for ananti-lock brake system for vehicles using the road surface frictioncoefficient μ according to a seventh embodiment in the presentinvention. If the driver steps on the brake in an emergency, the brakepressure is increased at the step 200. Then, at the step 202, whether ornot μ sharply increases is decided. If it sharply increases, it isdecided to be a hard brake and the ABS control starting at the step 203is performed. If μ decreases at the steps 204 and 205 irrespective ofthe brake pressure being on the increase, it is decided that the μ maxvalue available on these road surface has been passed, and at the step206, the μ value before μ decreases is stored in MAX_μ_HI and theassociated brake pressure is stored in MAX_P_HI. At the step 207, on thebasis of this MAX value, the lower limit of vibration range of the usualroad surface is calculated as MAX_μ_LO and the corresponding brakepressure is calculated as MAX_P_LO. At the step 209, if the currentbrake pressure is greater than MAX_P_HI, the decrease of the brakepressure at the step 210 is effected, with the processing returning tothe step 208. Further, if the current brake pressure is less thanMAX_P_HI, the processing goes to the step 211. At the step 211, if thecurrent brake pressure is less than MAX_P_LO, the brake pressure isincreased at the step 212 and the processing returns to the step 208.Further, if, at the step 211, the current brake pressure is greater thanMAX_P_LO, the processing returns to the step 208 without doing anything.Thereby, the brake pressure can be maintained between μMAX and thecalculated fixed lower limit by a minimum of control.

FIG. 10, 10A is a schematic flowchart showing a control method for ananti-lock brake system for vehicles using the road surface frictioncoefficient 1 according to an eighth embodiment of the presentinvention. The operation from the step 200 the step 207 is exactly thesame as the seventh embodiment, so that a description thereof isomitted. In the loop including the steps 300, 301, 302, the pressure isdecreased until the brake pressure is equal to MAX_P_LO. Then, in theloop including the steps 303, 304, 305, the pressure is increased untilthe brake pressure is equal to MAX_P_HI. Then, the processing returns tothe loop including the steps 300, 301, 302. Thus, the brake pressure canbe varied such that it is between the MAX value and the calculated fixedlower limit.

FIG. 11, 11A is a schematic flowchart showing a control method for ananti-lock brake system for vehicles using the road surface frictioncoefficient μ according to a ninth embodiment of the present invention.The operation from the step 200 to the step 305 is exactly the same asin the eighth embodiment, so that a description thereof is omitted. Whenthe processing has gone through the loop including the steps 303, 304,305, that is, if at the step 400 the current μ value is greater thanMAX_μ_HI stored at the step 206, it can be decided that the vehicle hasmoved to a road surface where greater μ can be obtained. The processingreturns to the step 203 where the ABS control was started, and it ispossible to seek the μMAX value available from this road surface.

FIG. 12, 12A is a schematic flowchart showing a control method for ananti-lock brake system for vehicles using the road surface frictioncoefficient μ according to a tenth embodiment of the present invention.The operation from the step 200 to the step 302 and from the step 303 tothe step 305 is exactly the same as in the eighth embodiment, so that adescription thereof is omitted. At the time of the step 500, the brakepressure is at the value of MAX_P_LO, and at the time of the step 207,the corresponding μ was MAX_μ_LO. If the current μ is smaller than thevalue of MAX_μ_LO, it is decided that the μ of the road surface hasdecreased, and after quick decrease of pressure at the step 502, theprocessing returns to the ABS control start point at the step 203. Atthe time of the step 501, the brake pressure is at the value ofMAX_P_HI, and the corresponding μ at the time of the step 207 was at thevalue of MAX_μ_HI. If the current μ is smaller than the value ofMAX_μ_HI, it is decided that the μ of the road surface has decreased,and after quick decrease of pressure, the processing returns to the ABScontrol start point at the step 502.

FIG. 13, 13A is a schematic flowchart showing a control method for ananti-lock brake system for vehicles using the road surface frictionforce F according to a seventh embodiment of the present invention, itbeing noted that the road surface friction coefficient μ has beenreplaced by the road surface friction force F. Since the method ofcontrol can be performed in exactly the same way as in the case of μ, adetailed description thereof is omitted.

Further, the control using μ shown in FIGS. 10 through 12 can bereplaced by the control using F as in the case of FIG. 13.

Method of Detecting Control Point in ABS

The invention will now be described with reference to embodiments shownin the drawings. FIG. 14 is a functional system diagram and FIG. 15 is atypical stress sensor versus output graph obtained when a vehicle isbraked hard leading to the locking of the wheel. As can be seen fromFIG. 15, when a vehicle is braked hard, for some time (after brake on)with sufficient friction force remaining on the road surface, as thecurve P (brake oil pressure P) rises, the curve F (road surface frictionforce F) and curve T (brake torque T) increase at the same rate ofincrease in proportion to the curve P. It is known that the proportionvalue (F/T) in the output values of the curves F and T in this period oftime is constant. However, when the friction force (dependent on thetire and road conditions) obtained from the road surface approaches thelimit, the brake torque represented by the curve T continues rising asusual in proportion to the brake oil pressure, but it is known that therate of increase of the road surface friction force represented by thecurve F decreases and then rapidly decreases as soon as the brake oilpressure exceeds a given pressure (the brake force corresponding to thelimit value of friction force obtained from the road surface).Therefore, in this period of time, the proportion value (F/T) in theoutput values of the curves F and T rapidly decreases.

Further, it sometimes happens that the value of the brake torque T ismuch greater than the road surface friction force F and with the roadsurface friction force detecting means 1 of FIG. 14 it would be verydifficult to detect the pure road surface friction force F. Therefore,usually it follows that with the road surface friction force detectingmeans 1, a substantial amount of brake torque T mixes in and is measuredas crosstalk. If, however, the proportion value (F/T) is used fordecision in control rather than using the detected value of the roadsurface friction force F, then$\frac{( {F + t} )}{T} = {\frac{F}{T} + \frac{t}{T}}$where t is the crosstalk component which has mixed in the road surfacefriction force F. Further, since t is a value proportional to the braketorque T, t/T is a constant. Therefore, in the case where F/T is used todetect the change therein so as to decide the control point, it ispossible to effect control of the type in which crosstalk component dueto the brake torque T is eliminated.

Thus, as shown in FIG. 14, immediately after braking, the detected valuefrom the road surface friction force detecting device 1 and the detectedvalue from the brake torque detecting means 2 are fed to the arithmeticmeans 3, where the F/T is successively calculated and the result is fedto the decision means 4. In the decision means 4, the ratio F/T ismonitored immediately after the braking and at the point of time whenthis ratio has suddenly decreased (the change has increased), it isdecided that the wheel is going to be locked. And this point in time isdetected as the point in time for the first pressure decreasing timingin ABS control and the detected signal is used to give a brake oilpressure decreasing instruction to the ABS control device 5; in thismanner, the ABS control to avoid the locking of the wheel is madepossible.

If the brake oil pressure P is increased (the oil is pressurized), thebrake torque T increases through the transmission delay in the brakesystem, and through the transmission delay the road surface frictionforce F increases. Generally, the region on the left-hand side of thepeak of the curve F in FIG. 15 is called the stable region, while theregion on the right-hand side of the peak of the curve F in FIG. 15 iscalled the unstable region. If the balance is maintained in the stableregion, even if the road surface friction force F changes owing tosending small stones flying, a force acts by which the original positioncan be instantly restored. However, if the change in the road surfacefriction force F takes place in the unstable region, it rapidly moves inthe wheel locking direction (to the right-hand side of the peak of thecurve F) or to the position in the stable region where balance can bemaintained.

Therefore, in a fifteenth embodiment the ratio F/T being constant (thefirst differential of F/T being 0) means stability in the particularposition (to the left-hand side of the peak of the curve F) in the graphof FIG. 15, which means that the brake oil pressure P and the braketorque T are balanced with each other. The ratio F/T being constant (thefirst differential of F/T being 0) means that for example, the brake oilpressure decreasing control when the brake oil pressure P is excessivelyincreased and moves to the unstable region moves it to the stable regionto provide the road surface friction force F which is proportional tothe current brake oil pressure P. This serves as a confirmation decisionallowing the next control to be performed upon completion of thepreceding control.

The time when the brake oil pressure P is being decreased is the timewhen it goes too far in the direction of instability to result in theF/T value being high, and the decision of stopping decreasing the brakeoil pressure can better be performed by referring to the preceding orfirst successful control. In the case of referring to the precedingcontrol, if the brake oil pressure P or the brake torque T which isproportional to P before the locking of the wheel is used to decide thepressure decrease stopping point, the optimum control oil pressure P maychange to result in erroneous control if there is a change of the roadsurface between the preceding time of control and this time of control(for example, from a dry asphalt pavement to a road wet with water).However, if the F/T value is used for making the decision, since the F/Tvalue does not almost change, the decision using the comparison with thepreceding value as described in relation to the sixteenth embodiment atthe completion of brake pressure decreasing control or the comparisonwith the first control value at completion of brake pressure decreasecontrol as in the seventeenth embodiment results in a less erroneousdecision and hence safer control decision; thus, such means is veryeffective.

As for the decision of stopping decrease of the brake oil pressure, ifthe delay in the control system is greater than the response required bythe control, fruitless pressure decrease would be performed worseningthe braking distance unless the control is effected before the F/T valuefully decreases to the target value. This time also, as in the above,the road surface friction force F is directly used; and in order not toresult in erroneous control owing to a change of the road surface,decision is made by at the point where the F/T value starts to move inthe target direction (the point where it starts to decrease) by usingthe F/T value as described in the eighteenth embodiment; in this manner,safer control can be effected.

In the case where the F/T value increases during the retaining of thebrake oil pressure P or during pressure control, this means that thepressure is moving in the wheel locking direction (to the right in thegraph of FIG. 15); therefore, the control is moved to decrease the brakeoil pressure P. This is not almost influenced by a change of the roadsurface since when the F/T value rises above that obtained during thefirst brake pressure decreasing control initiation of brake pressuredecrease control begins in accordance with a nineteenth embodiment.Thus, erroneous control can be effectively eliminated.

In the brake oil pressure retaining control, if the F/T is greater thanthe value obtained in the first or preceding brake pressure decreasingcontrol is stable, this means that the preceding control decision valueis too small, so that mild pressure control is effected in accordancewith a twentieth embodiment and the F/T value in continues the decrease,it is decided that it is moving in the unstable direction, so that thepressure decreasing control is effected in accordance with a twentyfirst embodiment of the invention. If the F/T value is small and stable,this means that the friction force F available from the road surface ison the increase, so that the mild pressure control or increase iseffected in accordance with a twenty second embodiment when F/T becomessmaller than the value obtained during the first brake pressuredecreasing control and is stabilized.

In a twenty third embodiment, according to the approximate equation ofmotion shown below${I \cdot \frac{\partial\omega}{\partial t}} = {{k\quad{1 \cdot F}} - {k\quad{2 \cdot T}}}$from the moment of inertia I of the wheel and F and T multiplied by theproportionality constants k1 and k2, respectively, the F-T value is avalue proportional to the wheel acceleration dω/dt. This can be used asa wheel speed sensor. Thus, there has been realized a wheel speed sensorcapable of real time sensing which has eliminated the drawback of beinglacking in response (the slower the sensing, the worse this situation)which is characteristic of the conventional gear-like wheel speedsensor. both the slip ratio control based on the wheel speed accordingto the prior art and the control decision based on F or μ according tothe present invention are made possible by a single sensor.

In all embodiments described above, F/T has been used, but substantiallythe same result can also be obtained by using μ/T. However, since μ=F/T,the load moving component of the vehicle is included in μ.

1. A method of controlling an anti-lock brake system for vehicleswherein when a vehicle having an anti-lock brake system mounted thereonhas emergency brake applied thereto, a change in road surface frictionforce or road surface friction coefficient due to the increased brakepressure is detected by a road surface friction force detecting deviceor a road surface friction coefficient detecting device, said methodbeing characterized in that to decide the optimum control start pointconcomitant with the increase of the signal value of road surfacefriction force F or road surface friction coefficient μ detected by theroad surface friction force detecting device or road surface frictioncoefficient detecting device, use is made of means for eliminatingdisturbance sources which impede such decision.